This application claims the priority of European patent document 00 113 025.6 filed Jun. 23, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a fiber composite with a piezoelectric sensor or actuator integrated therein.
In piezoelectric materials, such as quartz crystals or PZT-ceramics (PZT=lead-zirconate-titanate), an applied electric field results in elongations or contractions of the material. Piezoceramic actuators are therefore able to convert electric energy directly into mechanical energy and offer advantages, such as high actuating resolution, high actuating power and very short response times, while their size is small. This effect is reversible in the case of piezoelectric materials; that is, a time-variable mechanical elongation of such ceramics causes, a charge displacement between the electrodes which can, in turn, be tapped as an electric sensor signal. In combination with suitable sensors and control, actuator systems can be implemented which can automatically adapt (that is, are adaptive) to changed operating conditions.
Piezoceramic actuators and sensors are typically constructed as stack actuators, elongators and bending actuators. The former consist of stacks of thin piezoceramic disks which are elongated or shortened approximately linearly along the longitudinal stack axis under an exterior electric field. The two latter consist of thin ceramic plates which, as a rule, are flatly connected with a carrier structure and elongate the latter while an electric voltage is impressed and generate an electric signal when the structure is elongated. In an asymmetrical integration into the carrier structure, or in the controlling of a bimorph in opposite directions (bimorph=actuator or sensor consisting of at least two separately bonded and mutually insulated piezoelectric wafers which are arranged in two or more planes in a parallel and congruent manner above one another), by means of the actuators, bending moments can therefore also be induced. When such elongation is blocked, elongators and bending elements can be electrically controlled to transmit forces to the corresponding structure and, to a certain degree, increase its stiffness.
The induced elongations and curvatures, or forces and moments, of piezoceramic elongators and benders can be used for adaptive damping of structure vibrations and for the active shape control (that is, adaptation of the geometrical shape to the respective operating conditions). The combination of such elements with fiber composites (which, because of their excellent characteristics in comparison to conventional metallic materials, are increasingly used in highly stressed as well as highly precise structuresxe2x80x94particular in the aerospace field) creates the unique possibility of embedding such actuators and sensors on the basis of piezoceramic materials directly into the structure, instead of applying them only to their surface. As a result, adaptive structures of light weight and high stability are obtained, which can be cost-effective in future systems, for both manufacturing and in continuous use.
The significant advantages favor integration of the actuators/sensors in comparison to the application to the component surface, can be summarized as follows:
Higher effectiveness of elongation transmission;
capacity for integration under a higher mechanical (pressure) prestress;
protection against environmental and media influences
maintenance of aerodynamic contours with little or no influence on the surface quality of a component.
In the meantime, a number of important fields of application for piezoceramic actuators and sensors, including suppression of vibrations, reduction of sound or shape control of adaptive structures have partially been implemented in the form of demonstrators as well as products. These include modern spacecraft as well as civil and military systems, such as launcher rockets, satellites, helicopters, aircraft, road and rail vehicles, but also submarines and torpedoes.
The problems encountered in integrating piezoceramic actuators in a fiber composite will be explained herein using as an example the QuickPack(copyright) of the U.S. manufacturer Active Control Experts (ACX) [ACX], illustrated in FIG. 1. Currently, this is the only commercially available piezoceramic actuator/sensor which appears to be usable on an industrial scale because of its module-type configuration with feed lines, electrically insulating encapsulation and mechanical stabilization of the brittle piezoceramics. ACX QuickPacks(copyright) are flat thin piezoceramic actuators and sensors which generate forces and elongations which, to a first approximation, are proportional to the applied electric field, and which are utilized primarily in the plane of the actuator. They consist as a rule of one (unimorph) or two piezoceramic wafers (bimorph) which are arranged above one another, are insulated with respect to one another and separately controllable.
When the two wafers are controlled in opposite directions, they can function as benders. The standard actuators have one or two wafers arranged behind one another in a plane. In special applications, also up to five-layer modules with up to 2xc3x975 wafers per plane were manufactured [Hopk98], [Mans99], [Ditt99] and [Dxc3xcrr99]. Original ACX QuickPacks(copyright) are provided primarily for the application to a structure by means of an epoxy resin adhesive.
The negative characteristics which are normally associated with piezoceramic actuators (brittleness, problematic electric contacting and lack of electric insulation) are largely eliminated in encapsulated piezoceramic actuators by a special manufacturing process: The electroded piezoelectric wafers are glued to thin polyimide foils in the thickness direction. Electric strip conductors made of copper are, in turn, applied to the polyimide foils. The latter are therefore bonded and are electrically mutually insulated, and also insulated with respect to the environment, by an additional thermoplastic border (spacer). Charge transmission takes place without additional process materials or soldering points, purely by virtue of the mechanical contact between the copper strip conductor and the electroding. At one end of the actuator, the individual polyimide foils, together with the imprinted strip conductors, are guided together to form a type of flat band cable (tail), which, in comparison to the dimensions of the actuator, is narrow and provided with a standard connector. In addition to a further mechanical stabilization and slight pressure prestressing of the encapsulated piezoceramic actuator, the polyimide foil is used as an additional protection against outside influences, such a moisture or dirt. Thus, the brittle and thin piezoceramic wafers can be handled and processed relatively easily [ACX]. The QuickPack(copyright) actuators mentioned as an example of encapsulated piezoceramic actuators can be obtained in a series of different configurations. The tests, on which the above-described invention is based, are carried out by means of QP20N actuators and are verified on QP40N actuators.
This actuator type is permissible only for temperatures of up to maximally 100xc2x0 C.; at higher temperatures, parts of the plastic materials used for the encapsulating and gluing-together of the individual layers start to decompose; this generally leads to a massive delamination of the actuator, and to catastrophic destruction. However, modern high-capacity composite materials are generally manufactured at temperatures of up to 180xc2x0; sometimes, an additional cure cycle (post-cure) of components at still higher temperatures will be required.
The described tails of the QuickPack(copyright) actuators are incompatible with a structurally conformal integration in fiber composites, because guiding of the electric feed line out of a fiber composite component requires severing the cover layers on the actuator. The latter is necessarily accompanied by an intolerable reduction of the strength, destroying the advantage of a structural integration.
It is an object of the invention to provide a fiber composite structure with an integrated piezoceramic actuator or sensor whose electric feed lines can be guided out of the fiber composite with as few interferences as possible.
Another object of the invention is to provide such a structure in which the integration of the actuator or sensor leads to no significantly impaired mechanical characteristics.
Still another object of the invention is to produce such a fiber composite with an integrated actuator or sensor, in which particularly the structure and the quality of the surface of the fiber composite is to be maintained without any limitation.
These and other objects and advantages are achieved by the fiber composite according to the invention, in which the piezoceramic sensor or actuator integrated therein has feed lines for the sensor or actuator in the form of electrically insulated thin wires which extend out of the fiber composite perpendicular to the laminate layers, so that the fibers are not severed by the leading-out of the feed lines, but are only slightly pushed apart. This arrangement achieves the integration of the actuator or sensor into the fiber composite structure without a significant reduction of the strength characteristics of the component.
Within the component, the feed lines, in sections, can also be guided parallel to the laminate layers. As a result, the feed lines can exit the component at any point on its surface, independently of the installation site of the actuator.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.